3.24: Exothermic Reactions

This mushroom cloud was produced in a 1953 nuclear bomb test in Nevada. There’s no doubt that the explosion gave off a huge amount of energy. Although not as impressive as nuclear reactions, many chemical reactions also give off energy. These reactions are called exothermic reactions.

What Is an Exothermic Reaction?

All chemical reactions involve energy. Energy is used to break bonds in reactants, and energy is released when new bonds form in products. In some chemical reactions, called endothermic reactions, less energy is released when new bonds form in the products than is needed to break bonds in the reactants. The opposite is true of exothermic reactions. In an
exothermic reaction
, it takes less energy to break bonds in the reactants than is released when new bonds form in the products.

Energy Change in Exothermic Reactions

The word
exothermic
literally means “turning out heat.” Energy, often in the form of heat, is released as an exothermic reaction proceeds. This is illustrated in the
Figure
below
. The general equation for an exothermic reaction is:

Q:
You turn on the hot water faucet, and hot water pours out. How does the water get hot? Do you think that an exothermic reaction might be involved?

A:
A hot water heater increases the temperature of water in most homes. Many hot water heaters burn a fuel such as natural gas. The burning fuel causes the water to get hot because combustion is an exothermic reaction.

Combustion as an Exothermic Reaction

All combustion reactions are exothermic reactions. During a combustion reaction, a substance burns as it combines with oxygen. When substances burn, they usually give off energy as heat and light. Look at the big bonfire in the
Figure
below
. The combustion of wood is an exothermic reaction that releases a lot of energy as heat and light. You can see the light energy the fire is giving off. If you were standing near the fire, you would also feel its heat.

Summary

An exothermic reaction is a chemical reaction in which less energy is needed to break bonds in the reactants than is released when new bonds form in the products.

During an exothermic reaction, energy is constantly given off, often in the form of heat.

All combustion reactions are exothermic reactions. During combustion, a substance burns as it combines with oxygen, releasing energy in the form of heat and light.

Vocabulary

exothermic reaction
: Chemical reaction that releases energy because it takes less energy to break bonds in the reactants than is released when new bonds form in the product.